Renal calculous disease is a common disorder in this country which has been suggested to be on the rise. In 1950, only 0.95 per one thousand Americans were estimated as having the disease. By 1984, this number had risen to 1.64 per one thousand Americans. Boyce, W. H. [1979] "Epidemiology of Lithiasis in the United States," XVIIIth C. M. Congress of the International Society of Urology, Paris, Kongressbericht, Tome 1, pp. 79-86. A 1978 report by Sierakowski et al. estimated that about 12% of the U.S. populace will suffer from stone disease at least once in their lifetime (Sierakowski, R., B. Finlayson, R. R. Landes, C. D. Finlayson, N. Sierakowski [1978] Invest. Urol. 16:438-441). That estimate would be expected to have increased substantially in accordance with the increased incidence of kidney stone disease.
The most prevalent type of renal stone disease is of a calcareous and idiopathic nature, occurring mostly in white males. Therefore, it is known that incidence of kidney stone disease is associated with gender and race differences. Geographic and dietary factors can also affect incidence.
Unlike normal biomineralization processes (e.g., bone and tooth formation), calcium oxalate urolithiasis is a pathological process. Although this process has been extensively studied, adequate therapy for the disease can be a highly complex and uncertain subject to the average medical practitioner. See Thomas, Jr., W. C., [1976] Renal Calculi: A Guide to Management, Charles C. Thomas, Publisher, Springfield, Ill., 177 pp. In addition, kidney stone disease can be an extremely painful experience for the patient. Passage of stones can cause renal colic, destruction of kidney tissue from mechanical irritation by the stone, as well as back pressure from obstruction of urinary flow. An increased risk of infection also accompanies the presence of any foreign body.
Crystallization within the urinary tract occurs opportunistically and quite freely. Formation of smaller crystallites is a normal renal function for eliminating calcareous stone salts. Abnormal conditions presumably result from uncontrolled crystal agglomeration and/or cellular attachment/retention of crystals. Stone disease arises when crystallite attachment is not blocked within the urinary tract. Consequently, further biomineralization and accretion of other cell debris and solutes create flow-obstructing kidney stones. Although certain factors, e.g., uncontrolled biosynthesis of oxalate (hyperoxaluria), elevated phosphate levels (phosphaturia), and excessive dietary intake of oxalate-rich foods can exacerbate stone formation, it remains an idiopathic disease.
The prevalence of kidney stone disease arising from pathological calcium oxalate biomineralization contributes substantially to the cost of health care in the United States. For example, costs for hospitalization and surgery or extensive outpatient extracorporeal shock-wave lithotripsy treatment of approximately 500,000 U.S. residents annually has been estimated at $3 billion. These figures exclude lost occupational time during treatment or recuperation. Davis et al. (Davis, G. K., N. B. Cummings, B. Finlayson, J. L. Meyer, M. J. V. Smith [1974] "Urolithiasis," in Geochemistry and the Environment, Natl. Acad. Sci., Washington, pp. 133-138) put the number of lost working days at nearly 15,000,000 annually in this country alone, which translates to around 56,000 person-years of productivity lost each year. An even more telling expression of the economic impact is that a work force of 56,000 corresponds to the productivity of an entire city of about 130,000 to 150,000. Thus, it is clear that a simple, accurate, and reproducible method for detecting or treating such conditions, even in advance of stone formation, is a valuable contribution to the management of kidney stone disease.
Previously, methods for detecting kidney stone formation or treating kidney stone disease have been described, but are quite different from the subject invention. For example, U.S. Pat. No. 5,137,722 describes an extract and pharmaceutical composition for treatment of calcium oxalate stone disease. The extract is purified from the plant Eriobotrya japonica. The extract does not consist of any citrate-containing compounds.
U.S. Pat. No. 4,399,003 describes a method and kit for diagnosing a patient's proneness to develop calcium oxalate-type kidney stones. The method comprises measuring the rate of decrease of calcium ion concentration in the patient's urine sample as compared to a reference standard of normal urine. This method differs substantially from the diagnostic method described hereinbelow, which does not directly measure calcium ion concentration in the patients' urine.
U.S. Pat. No. 4,888,182 also describes methods and compositions for the treatment and prophylaxis of calcium renal stones. By contrast, the subject invention comprises a novel compound and methods of use which are clearly distinguishable from the citrate salt and methods of using that ionic salt as described in the '182 patent.
Except for a small fraction of stone-formers (e.g., individuals with primary hyperoxaluria, those with renal tubular acidosis, and others having chronic hypercalcemic disorders), there has been no clearly defined single criterion, or uniform set of criteria, that can be characterized for the much larger group of idiopathic kidney stone-formers, which are addressed by the subject invention.
A few other potential diagnostic tools for discerning urolithiasis have been described in the scientific or medical literature. Illustrative are the work of Zhmurov (Zhmurov, V. A. [1991] Urol. Nefrol-Moscow May-June (3):12-15), who found altered phospholipid composition in the urine of stone-formers, and Azoury et al. ([1990] Urolog. Res. 18:7-11) who described the use of nuclear magnetic resonance proton-relaxation-rates (PRR) to distinguish healthy and stone-former urine samples. The method of Zhmurov et al. has the obvious disadvantage of analyzing changes in complex patterns of phospholipid composition. With regard to the Azoury et al. method, the high cost of nuclear magnetic resonance instrumentation is a major contributing factor to its general unavailability in clinical laboratories.
The techniques described either by Zhmurov et al. or Azoury et al. have not been instituted as a commercial diagnostic measure. More fundamentally, the changes reported by Zhmurov et al. and Azoury et al. may relate to secondary pathophysiologic responses to the presence of kidney stones or to the absence of dicitrate cyclic diester (see discussion below), and they may not be generally applicable prior to the occurrence of stone disease.
The type of calculus evidenced in any particular patient is due primarily to a urinary deficiency of inhibitors normally present to help suppress crystal formation. Several compounds have been proposed as contributing factors in the inhibition of crystal formation. For example, Schreier et al. reported that about one-half of the inhibitory activity for calcium oxalate crystal growth in urine was due to compounds smaller than 5000 daltons, and that a portion of the activity had been shown to be due to pyrophosphate and citrate. Large molecular weight inhibitors in three families, e.g., glycosaminoglycans (GAGs), ribonucleic acids (RNAs), and glycoproteins, accounted for approximately 40% of total inhibition. The remainder of inhibitory activity was due to compounds which were chemically undefined. Thus, a variety of compounds have been suggested as potential inhibitors of kidney stones (Schreier, E. E., K. E. Lee, J. L. Rubin, P. G. Werness, L. H. Smith [1979] "Macromolecular Inhibitors of Calcium Oxalate Crystal Growth and Aggregation in Urine," In Oxalate in Human Biochemistry and Clinical Pathology (G. A. Rose et al., eds.), London, The Welcome Foundation, pp. 22-610).
It has been known for decades that urine can prevent cement from hardening properly. One of the current inventors described isolation, using column chromatography, of a fraction from urine which appeared to inhibit setting of cement. A citride compound was synthesized which reduced the ability of cement to harden, but it was not determined what the exact structure of the compound was. The compound was determined to be distinct from the water soluble di-, tri-, or tetramethyl citrate compounds isolated from pineapple in the course of preparing vitamin C, as described nearly sixty years ago. See Bose, P. K., S. N. Bhattacharyya (1936) Science and Culture 2:162. Moreover, it was undetermined what the role of the compound was in the inhibition of hardening of cement and whether there was any relation to kidney stone formation (Thomas, Jr., W. C. [1988] Md. Med. J. 37:861-862). Nor was it known whether this compound corresponded to a natural inhibitor or whether the compound was a single chemical species. Furthermore, there was no evidence then available to indicate that calcareous stone disease could be reliably linked to the absence of any single metabolite. The speculative nature of these studies proposing a role for inhibitors of calculus formation is evidenced by subsequent studies which continued to suggest that proteins, especially glycoproteins, played an important role in the inhibition of calcium oxalate crystal formation (Coe, F. L., Y. Nakagawa, J. H. Parks [1991] Am. J. Kidney Disease 17:407-413). However, epidemiological studies of certain substances, e.g., citrate, phosphate, pyrophosphate, glycosaminoglycans, nephrocalcin, Tamm-Horsfall proteins, and uropontin, showed that none of these substances can be used as an unambiguous, single-determinant marker for calcareous stone disease.
Accordingly, the development of prospective diagnostic techniques to identify those patients who are predisposed to developing kidney stones is needed to provide a means for mitigating these costs through the exercise of preventative practices by those predisposed patients. Therefore, the discovery of a singular, unambiguous marker as described hereinbelow is a substantial contribution to the art.